I recently needed to upgrade a project from Hibernate 3.6.9 to 4.1.3. Upgrading the code went fairly smoothly. However, upgrading the build turned out to be more difficult. The reason was the build was using hibernate3-maven-plugin to create an SQL schema file. The plugin, as its name suggests, supported only Hibernate 3. The main issue was that the plugin's code tried to use Hibernate classes which were relocated to different packages in the 4.x version. I tried updating the plugin from version 2.2 to 3.0, but it didn't work.